There has been known a waveguide slot array antenna apparatus in which a large number of slots parallel to the tube axis are alternately arranged at intervals of about ½ intra-tube wavelength with respect to the center line of a waveguide wide plane in the tube axial direction of the waveguide. Because an electric field is generated in the width direction of the slot, the polarized wave plane of the antenna is orthogonal to the tube axis.
Meanwhile, a waveguide slot array antenna having the polarized wave plane in a direction oblique to the tube axis of the waveguide is disclosed in, for example, Patent Document 1. In the waveguide slot array antenna, slot elements are alternately arranged at intervals of about ½ intra-tube wavelength in the tube axial direction across the center line of the waveguide wide plane, and the respective slot elements are inclined at given angles with respect to the tube axis, to thereby radiate linearly polarized waves in a direction oblique to the tube axis.
Patent Document 1 discloses an arrangement position of the slots and the inclined angles of the slots, but neither discloses nor suggests the selection of the length and width of the slots. In particular, the length of the slots influences the resonance characteristic and the excitation distribution of the waveguide slot array antenna, and its selecting method is important.
Patent Document 1: JP 9-64637 A    Patent Document 2: JP 2001-196850 A (FIG. 4, FIG. 5)